Lowering and/or raising a string of tubular equipment into a subsea borehole is a critical operation during drilling, or when e.g. preparing and/or closing a well for production. Such strings of tubular equipment may comprise casing and/or liner pipes placed in the borehole for stabilization of the borehole, protection of the formation or for optimizing a drilling operation. In order to reach larger depths, a string of casing and/or liner pipes may be suspended from a so-called landing string during any such lowering and/or raising operations. The operations require using the string building, lifting, drilling mud handling, and rotating functions of the facilities around the well centre. With boreholes achieving larger and larger depths, and with offshore drilling operations being performed where the top of the borehole is located on the sea floor at greater and greater depths of the sea, ever longer strings of tubular equipment have to be placed into the borehole or recovered therefrom. However, as the strings of tubular equipment to be handled during such operations get longer and longer, the total weight of the combined string may easily exceed 1000 metric tons, thereby reaching the load rating limits of much of the equipment in the configurations that are typically available on an offshore drilling rig.
One load limiting component is the top-drive commonly used on modern drilling rigs, which in addition to driving the rotation of e.g. a drill string during a drilling operation often also provides numerous other functionalities, such as a so-called link-tilt function used for an efficient and at least partially automated building of a string of tubular equipment, and/or mud handling functions for e.g. filling drilling mud into the string of tubular equipment as the tubular equipment is lowered into a borehole while being suspended in a load bearing device attached to the top drive. Such functions are typically a part of the so-called pipe handler hanging under the top drive. For the purpose to the present invention this pipe handler is considered an optional part of the top drive. The pipe handler typically uses interchangeable bails and pipe elevators to handle different types of tubulars. The top drive is typically suspended from a load carrier of a hoisting system, which provides the actual lifting functionality. The top drive is thus a load carrying link between the load carrier of the hoisting system and the heavy tubular string. In such a set-up, the safe working load rating of the top drive is typically the limiting factor and is often used as a design specification for designing the load capacity of the remaining components of such a system. While top drives with larger and larger SWL ratings become available on the market, these tend to be over-dimensioned for most of the primary applications for which top-drives are used in practice, and are therefore much more inefficient than a top-drive that is properly dimensioned for these primary applications.
Other load limitations may be inferred from e.g. the hoisting systems and the support structure supporting the hoisting system.
One object of the present invention is to provide a heavy duty system for lowering and/or raising tubular equipment during offshore drilling operations, which allows overcoming such load limitations in an efficient manner. Another object is to provide a cheaper, faster and/or more reliable rig where two hoisting systems of lower capacity (typically cheaper and/or faster relative to systems with a higher load rating) can work individually on most parts of a well and optionally provide redundancy and/or improved efficiency by having two hoisting system. For parts of the well where the drilling operation which may exceed the load capacity of the individual hoisting systems (such as running heavy casing) the hoisting systems may be arranged to perform this job in cooperation.
According to a first aspect of the invention this object is achieved by an offshore drilling rig configured for lowering and/or raising a string of tubular equipment into a subsea borehole as defined below. According further aspects, the object is achieved by an assembly, a connecting tool and a bail section for use in an offshore drilling rig, and a method of lowering and/or raising a string of tubular equipment into a subsea borehole through a joint operations well centre in a drill deck of an offshore drilling rig, as further detailed below.